A micro-wormhole
Interior of micro-wormhole
A micro-wormhole is a wormhole in an advanced state of collapse. A micro-wormhole could have originally been of a much greater size, but over time degraded and shrunk to a smaller size.
In 2371, the USS Voyager discovered a micro-wormhole 30 centimeters in diameter. The wormhole led from the Delta Quadrant to Romulan space (in the past). (VOY: "Eye of the Needle")